Misfires return

[pkufeldt]

1999 Suburban K1500 5.7L 133k miles

I have been tracking down a long standing misfire problem with my suburban and have done many repairs and tests along the way in an attempt to fix. Here is the sequence:

1. P0300 and P0306 DTCs appear
2. Arbitrarily replaced plugs, wires, disty rotor and cap.
3. Compression tested all cylinders - everything looked fine
4. Misfires got progressively worse creating blinking MIL - I assume I started seriously damaging my cats
5. Replaced fuel filter and CSFI injector with a new MFI injector - introduced intake leak as fuel meter body o-ring didn't seat properly
6. Misfires continue
7. Performed a leak down test of all cylinders - everything < 6%
8. P0430 DTC appeared
9. Got a bluetooth OBDII reader and using Car Gauge Pro determined misfires on all cylinders but cyls 6,4,3 were much higher in the thousands, CMP retard too high (4+), LTFT too high (20%+) and B2S2 following B2S1 in closed loop - I evidently destroyed the Bank2 cat.
10. Replaced disty, got CMP retard to 0-.1, cam gear was seriously worn - knife edges
11. Misfires almost completely resolved - still maybe 1 or 2 on one cyl at high speed, never enough to record in the history. Felt smooth and P030x DTCs were resolved.
12. Found intake leak I created in #5 using a smoke test and repaired. LTFT lowered but still too high with 10-15% still being registered.
13. Tested fuel pressure, when fuel pump is active it is at 60-62psi but when off settles back to 50psi and is regulated there. That is at least 5psi too low.
14. O2 sensors are the originals, so I bought new O2 sensors. I bought new cats but have not installed them yet.
15. I replaced only one upstream sensor (B1S1) on the old cat to see if it helped the LTFT; it did lower it, but now misfires have returned, mostly at idle. Still too few to record in the history <10 per MF cycle.

So here's the question, could the lowering of the FTs have exposed another problem, namely that the fuel pressure is below specs? My understanding is that the lower FTs create shorter injection times - my guess is that at lower rpms I might be starving a cyl into misfiring. Or should I be looking at something else?

Also don't want to install the new cats until I fully resolve the the new misfires.

 

[Schurkey]

Your numbering in my quotes is screwed-up. The software restarts the numbering, and it shouldn't.

1999 Suburban K1500 5.7L 133k miles

1. Found intake leak I created in #5 using a smoke test and repaired. LTFT lowered but still too high with 10-15% still being registered.
2. Tested fuel pressure, when fuel pump is active it is at 60-62psi but when off settles back to 50psi and is regulated there. That is at least 5psi too low.

Pump running, engine off, you have 60--62 psi?
Pump off, engine off, pressure drops to 50 psi and holds there? How long does it take to drop from 60 to 50?

What is the pressure with the engine running at idle?
What is the pressure with the engine running at higher speed/higher load?

1. I replaced only one upstream sensor (B1S1) on the old cat to see if it helped the LTFT; it did lower it, but now misfires have returned, mostly at idle. Still too few to record in the history <10 per MF cycle.

Consider replacing the other upstream O2 sensor. See what happens.

could the lowering of the FTs have exposed another problem, namely that the fuel pressure is below specs? My understanding is that the lower FTs create shorter injection times - my guess is that at lower rpms I might be starving a cyl into misfiring. Or should I be looking at something else?

I'd be using the scan tool to verify EVERY sensor and computer output. In particular, do you have a stable RPM signal at idle, or does it suddenly drop to zero rpm during the misfire? If so, a weak crank sensor is a possibility.

don't want to install the new cats until I fully resolve the the new misfires.

Wise.

 

[pkufeldt]

Your numbering in my quotes is screwed-up. The software restarts the numbering, and it shouldn't.


Pump running, engine off, you have 60--62 psi?
Pump off, engine off, pressure drops to 50 psi and holds there? How long does it take to drop from 60 to 50?

What is the pressure with the engine running at idle?
What is the pressure with the engine running at higher speed/higher load?


Consider replacing the other upstream O2 sensor. See what happens.


I'd be using the scan tool to verify EVERY sensor and computer output. In particular, do you have a stable RPM signal at idle, or does it suddenly drop to zero rpm during the misfire? If so, a weak crank sensor is a possibility.


Wise.

Just re-looked at my fuel pressure test video's to get accurate numbers:

Pump running, engine off: 61psi
Pump off, engine off, immediately drops to 51psi and holds (there might be a small loss 51.8->51)
Running at idle: 50-51psi
Running at higher rpm (>2k): settles down to 50psi and holds
Let off the gas, as it goes back to idle, I see as low as 49psi but then rises back to 50psi and holds.

Will replace the other upstream tomorrow and test it.

Here is a graph showing RPMs to total misfire count (current cycle) during an idle to nearly 2k and back down to idle test. Getting 3 RPM readings per second and never seeing a weird reading. Collected nearly 20 minutes of data and saw < 8 misfires total and never saw a 0 RPM (other then when the engine was off .

You must be registered for see images attach

 

[pkufeldt]

Just an update, I re-installed the old possible lazy O2 sensor which raises the FTs and the misfires disappeared. So I am still thinking it might be the fuel pump.

 

[pkufeldt]

So I am still struggling with the misfire problem. Since the last message I have replaced all of the O2 sensors, buttoned up the intake leaks and FTs have lowered. I also replaced the fuel pump. I still have the same fuel pressure issue, fuel pump running (power on engine off) it clocks in at 61-62psi - in spec. But as soon as it stops pumping and I mean instantaneously it drops to 51psi - out of spec. It does not drop any further over the next few minutes. When running idle it is at 49-50psi. I followed the service manual diagnostic and replaced the fuel filter with a stop valve, brought it to pressure, shut the valve and took a pressure reading at the normal throttle meter body inlet, 56-58psi - in spec. The isolation the valve creates tells me that problem is in the supply line from sending unit to the fuel filter.

This is the big question: What on earth could cause an instant drop like that?

Could this out of spec pressure be causing misfires under load?

10 psi drop video

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[West 1]

I am guessing you have the newer Spider installed? The newer spiders have a new pressure regulator installed. It holds pressure lower at around 50 PSI. I have one in my garage now that does the same. Turn key on, 62 PSI, start engine drops to 52 PSI, turn key off, 62 PSI. A new pump won't change the PSI running it is controlled by the Spider Pressure relief valve.

On this truck I found the miss was on #3, it had been in 4 shops and each had the owner spend a lot of money but did not fix his issue of #3 misfires. I found it to be a sticky #3 exhaust valve. Carbon build up was holding the valve open but only at certain times. Towing, pulling large hill at first. Eventually it started mis firing when it reached full temperature making it easier to find. A leak down test showed it to be the #3 exhaust, with the valve cover off I could pry up on the #3 exhaust and the leak would go away but the heads are now off and in the shop getting a full valve job. This is pretty common on the 5.7L Vortec engines but hard to find. Everyone starts with the new Spider, Distributor, cap, rotor, plugs and wires. Sometimes even the ECM is changed.

Hope this helps.